Recently digital broadcast for transmitting/receiving image data and the like as digital data has been attracted attention. The digital broadcast has an advantage that a multiplicity of program data (hereinafter, referred to as program) can be transmitted as compared with analog broadcast in the case that the same transmission path is used. The reason for this greatly depends on a fact that image data to be transmitted can be compressed. As a method for compressing image data, for example, a bidirectional predictive encoding method adopted in an MPEG (Moving Picture Experts Group) standard has been employed. In the bidirectional predictive encoding method, three type of encoding are performed, which are intra-frame encoding, intra-frame forward predictive encoding and bidirectional predictive encoding, respectively. Images formed by the respective encoding methods are called, I picture (intra encoded picture), P picture (predictive encoded picture) and B picture (bidirectionally predictive encoded picture), respectively.
In a compression-encoding of an image for digital broadcasting, high image quality must be maintained while the quantity of compression-encoded data (an amount of bits) is suppressed to at most a transmission capacity of the transmission path.
A technique called “statistical multiplexing” may be employed to transmit more programs to a transmission path having a predetermined transmission capacity. The statistical multiplexing technique is a technique in which the transmission rate of each program is dynamically changed to transmit more programs. In the statistical multiplexing technique, the transmission rate of each program whose image quality does not deteriorate even if the transmission rate thereof is reduced is reduced to allow more programs to be transmitted.
Referring to FIGS. 12 and 13, the statistical multiplexing technique will further be described.
FIG. 12 shows an example of assigned code rates to respective programs when the programs are multiplexed by conventional fixed rates, wherein the axis of ordinate denotes the assigned code rates to the respective programs, while the axis of abscissa denotes time. As shown in FIG. 12, assigned code rates to respective programs, such as a weather forecast and news and a drama, which are multiplexed are kept at code rates assigned as initial values, which have not been changed according to time elapse. The code rates which are assigned to the respective programs as the initial values are assigned such that deteriorations of image quality in portions (time) of the respective program whose deteriorations of image quality are conspicuous is in allowable ranges. As a result, excessively high code rates are assigned to portions except for the portions in which the deteriorations of image quality are conspicuous.
FIG. 13 shows an example of assigned code rates to the respective programs when the code rates assigned to the respective programs are dynamically changed and multiplexed by using the statistical multiplexing technique, wherein the axis of ordinate denotes code rates assigned to the respective programs, while the axis of abscissa denotes time. The statistical multiplexing technique uses a fact that portions (time) of programs whose deteriorations of image quality in the respective programs are conspicuous unusually occur simultaneously. Since there are many cases where portions of a program whose deteriorations of image quality are conspicuous, correspond to portions of the other programs whose deterioration of image quality are inconspicuous even if the code rates thereof are reduced, a great code rate can be assigned to the program whose deteriorations of image quality are conspicuous by reducing the code rates of the other programs. In an example shown in FIG. 13, when a portion of drama (1) is conspicuous in deterioration of image quality and portions of news and drama (2) are inconspicuous in deterioration of image quality at time P, the code rates of the news and the drama (2) are reduced to assigne the drama (1) additionally with the code rate corresponding to the code rate thus reduced. Thus, employment of the statistical multiplexing technique enables a larger number of programs than usual to be transmitted.
A bit rate assigning method for determining bit rate amounts as code rates assigned to respective programs is an important factor for the statistical multiplexing technique to determine image quality or the like. As a representative technique which has conventionally been suggested as the bit rate assigning method there is a technique that quantizing steps for respective programs are monitored and feedback-controlled such that the quantizing steps are made equal to all of the programs or are balanced in accordance with weightings previously set to the respective programs. In the feedback control, since after completion of encoding of an image the next bit rate is determined in accordance with the quantizing step applied for encoding the image, if a picture is rapidly changed to a complicated picture because of scene change or the like, response delay is caused, which results in a problem in that image distortion excessively takes place.
On the other hand, to solve the problem of the response delay in the control system in which the feedback control is performed, the applicant of the present invention has proposed a technique called “feedforward control” in which encoding difficulty representing encoding difficulty in encoding regarding an image to be encoded is obtained in advance and the bit rate for each program is determined in accordance with the encoding difficulty. The feedforward control technique is realized by distributing the total bit rate after multiplexing to the respective programs in accordance with the ratios of encoding difficulty data basically obtained from the respective programs prior to the encoding processing. The distribution of the bit rate to the respective programs in accordance with the ratios of encoding difficulty data is determined by a proportional distribution expressed by the following Equation (1):Ri=(Di/ΣDk)×Total_Rate  (1)where Ri denotes a bit rate of an i th program, Di denotes the encoding difficulty of the program per unit time, Total_Rate denotes a total bit rate and Σ denotes the sum in the case of k=1 to k=L (L is the total number of the programs).
Alternatively, the distributions of the bit rate to the respective programs is determined by the following Equation (2) by adding the weighting coefficient Wi to the respective programs:Ri={Wi×Di/Σ(Wk×Dk)}×Total_Rate  (2)
The proportional distribution of the bit rate in accordance with the encoding difficulty, however, has the following problems.
(1) The proportional distribution in accordance with the encoding difficulty is not necessarily optimum for the visibility characteristic for a human being. For example, in the case of a simple picture in which the encoding difficulty is reduced, the bit rate is suppressed low and, human visual characteristic tends to find a distortion easily, on the contrary, in the case of a complicated picture, human visual characteristic tends not to find a distortion easily. Therefore, if the total bit rate is simply proportionally distributed in accordance with the encoding difficulties for the respective programs using one equation to all of the programs, an amount of bits is excessively distributed in a program of complicated picture, on the contrary, in a program of a simple picture in which an amount of bits is reduced, a distortion resulted from encoding is conspicuous so that an audience feels uncomfortable.
(2) In general, a digital broadcast station using the statistical multiplexing method bills companies, which present programs, to correspond to the amount of occupation of the bit rates. In the foregoing case, the digital broadcast station is required to previously assign an average bit rate to correspond to the budget of each company for presenting the program. However, a system using the statistical multiplexing method encounters dynamic change in the bit rate. Thus, accurate billing to correspond to the budget has been inhibited because an accurate average bit rate cannot be obtained.